Source for the gif-generator:

#!/usr/bin/perl
# micro-one-dot-gif. 
# 34 bytes non-transparent
# 42 bytes transparent.
# (c) magnus@bodin.org 1998-10-16 
# Freely distributable with the all comments preserved
# including copyright notice.
# e.g. http://x42.com/cgi-bin/gifdot.pl/ff00ff.gif
# returns magenta 1x1-pxl gif 
# (if it's transparent, then the color is just for show)

use strict;
binmode stdout; # necessary on NT
my $RGB='ffffff'; $RGB = $1 if $ENV{PATH_INFO} =~ /^.([\da-f]{6})/i;
my $g .= "0100010080FF00";
# Add global color map. Must be at least 6 bytes.
$g .= $RGB.'000000';
# Add Graphic Control Extension (uncomment to make the picture transparent)
#$g .= '21f9040100000000';
# Add Image Descriptor
$g .= '2C000000000100010000';
# Add Table Based Image Data
$g .= '020100003B';
# Pack the picture. 
$g =~ s#(..)#pack "C",hex $1#eg;
# http mock-up
print "Content-type: image/gif\012\012";
print "GIF89a$g";